Chimeric genes encoding sulfur-rich proteins have been transferred to plants for the purpose of increasing he sulfur amino acid content of the seed protein. For example, the transfer of a sunflower (Helianthus annuus) seed albumin (SSA) gene construct to Lupinus angustifolius in order to enhance the sulfur amino acid content and the nutritive value of the seed protein has been reported (Molvig et al. 1997). Plants such as canola (Altenbach et al., 1992), soybean (Townsend and Thomas, 1994) and narbon bean (Saalbach et al., 1995) have been transformed with genes encoding a methionine-rich protein from Brazil nut. In these cases, enrichment of seed protein with sulfur amino acids has been reported.
It has been found that transgenic soybeans containing the Brazil nut protein (BNP) at a level approximately equal to 4% of total seed protein had reduced levels of some endogenous sulfur-rich proteins, for example a storage protein, glycinin, and the Kunitz trypsin inhibitor (Townsend and Thomas, 1994). These effects on seed storage protein composition are similar to those which accompany sulfur stress in pea and lupin seeds. In the cases of peas and lupins, seeds grown under conditions of sulfur limitation contain decreased amounts of the storage proteins that contain sulfur amino acids, and increased amounts of storage proteins with little or no sulfur amino acids (Chandler et al, 1984, Blagrove et al., 1976). In the case of the transgenic soybean, the methionine-rich BNP was made at the expense of endogenous sulfur-containing compounds.
Similarly, it has been reported that transgenic narbon beans expressing BNP do not contain increased levels of total sulfur in comparison to non-transgenic narbon beans (Muntz et al., 1997). This indicates that also in the narbon bean, a new sulfur sink causes re-routing of sulfur away from endogenous compounds. A significant part of the seed sulfur of narbon bean exists in the form of the dipeptide, γ-glutamyl cysteine (GEC). Preliminary results showed that GEC was reduced in transgenic narbon beans expressing BNP, and it has been proposed to exploit this strategy for reducing the content of the unpalatable GEC in narbon bean in order to increase its utilisation in animal feeds (Muntz et al., 1997).
Notwithstanding the development of seeds rich in sulfur-containing proteins, the nutritive value of storage organs in general and seeds in particular is not dependent exclusively upon their sulfur content and many other factors, including total protein content, oil content and composition, fibre content and content of anti-nutritional proteins such as protease inhibitors, may influence the nutritive value of storage organs intended for human and/or animal consumption. Additionally, the production of storage organs having altered oil content and composition, which oils have improved industrial utility, nutritive value, human and/or animal health properties or consumer appeal, is particularly desirable. There is currently no single method available for simultaneously improving a wide range of nutritional attributes of storage organs.